Phase-shift keying is commonly used to convey information in digital form over a transmission medium. The demodulation of signals encoded using this method requires the multiplication of the received signal with a replica of the carrier, the signal that would have been received in the absence of modulation. This replica must have the correct frequency and phase to within a fraction of a cycle. In practice synchronisation is difficult to achieve over long or imperfect transmission paths. Maintenance of synchronism may be made more difficult if the path is time variant as a result of the motion of the transmitter and/or the receiver or if the characteristics of the transmission medium fluctuate.
Conventionally various methods are available for obtaining the required synchronising information. Firstly, a pilot carrier signal may be transmitted and superimposed on the phase-shift keyed signal and this pilot carrier then extracted at the receiver and used to synchronise the local oscillator which generates the carrier signal at the receiver. Alternatively, a phase-locked loop which locks onto either the phase-shift keyed signal or onto a pilot carrier may be used at the receiver to drive the phase difference between the received signal and the local oscillator of the receiver to zero. In general these methods are implemented using analogue techniques.
European Patent Application No. 84300682.6 does disclose a system in which decoding takes place in the digital domain. In this system a digital representation is produced of a signal received after transmission over an n-state phase-shift keyed data transmission system, a digital representation of a carrier signal is produced, and then the transmitted data is recovered using data processing techniques. However, the digital representation of the carrier signal is produced simply by a digital implementation of the commonly used analogue techniques involving the use of a local oscillator. Accordingly this method has the same disadvantages as the analogue techniques.